1. Field of the Invention
This invention relates generally to a motor control strategy and, more particularly, to a motor control strategy for a motor in an active front steering system for a vehicle that prevents torque ripple from being coupled to the hand-wheel of the vehicle.
2. Discussion of the Related Art
Hydraulic power assist steering systems for vehicles typically employ a hydraulic pump coupled to a hydraulic steering gear of the vehicle. The direction and magnitude of the power assist provided by the steering system is determined by a valve that is actuated by a torsion bar provided between the steering hand-wheel and the fixed end of the steering valve. Steering system connections in a conventional automotive vehicle are typically provided by direct mechanical linkages between a steering device and the front wheels of the vehicle. For example, in a rack and pinion steering system the rotation of the steering wheel rotates a pinion gear which is engaged with mating notches or teeth in a rack portion of the steering system. Tie rods connect the rack portion to the wheels, so that any rotational motion of the steering wheel ultimately rotates the road wheels to a resultant steering angle, which may vary depending on the steering ratio provided by the steering system. Other steering linkage designs may be used instead of a rack and pinion design, such as worm gears used in a re-circulating ball steering system. In either example, however, all control linkages are purely mechanical in nature.
In a by-wire steering system, an electronically controlled steering actuator, typically a brushless DC permanent magnet synchronous motor, is positioned on or in proximity to the front drive axle, and one or more transducers or other sensors measure or detect the steering request or input to a steering device, usually configured as a steering wheel. The sensors can measure a steering angle at the steering device, and/or a rotational force or torque applied to the steering device by an operator of the vehicle. The input signals representing these measured or detected values are then transmitted electrically to a steering actuator, which executes a steering maneuver in response to the signals. A by-wire steering system can be configured as a limited by-wire system in which a conventional mechanical steering linkage is retained, and in which an electronic steering signal is selectively used to augment or modify a steering response in the steering system. That is, if the motor is commanded with a positive or negative steering command, the commanded angle is added to or subtracted from the rotation of the hand-wheel after being reduced by an appropriate gear ratio, and is applied to the pinion gear.
Known controllers for the brushless DC motor driven AFS systems typically utilize commutation logic for the three motor phases based on three position sensors spaced apart at 60° or 120° electrical. The motor phases are switched whenever a transition of a position sensor is detected. Alternatively, the motor phases can be detected using a high resolution encoder. The motor phases typically have trapezoidal induced voltages and are controlled by applying rectangular current signals of 120° duration. The closed loop position control typically utilizes a proportional-integral-derivative (PID) control to determine the magnitude and phase of the motor command. Alternatively, the closed loop position control can be determined by proportional-derivative (PD) or proportional-integral (PI) control. U.S. patent application Ser. No. 11/560,876, titled Methods and Apparatus for an Active Front Steering Actuator, filed Nov. 17, 2006, assigned to the assignee of this Application and herein incorporated by reference, discloses a system where the motor command is determined by PID control when the angle error is above a predetermined maximum value and a hold mode when the angle error is less than a predetermined minimum value.
This process works well when the target angle is constant. However, any significant change in the target angle requires a transition to PID control. Transitions between a hold mode and a PID control mode create discontinuities in the PWM duty cycle command from the controller and cause torque disturbances, ripples and oscillations on the hand-wheel. These discontinuities are the result of one phase of the motor turning off at the same time that another phase of the motor was turning on, where there was an overlap of the on signals of the phases as a result of the trapezoidal shaped voltage signal and discontinuities in the rectangular shaped current signal. Each time there is a transition from one phase to another phase in the motor, there is a corresponding dip or ripple in the amount or torque produced by the motor, which is transferred through the steering column as a pulse. This problem cannot be easily resolved by calibration.